1. Field of the Invention
The present invention relates to a sheet supply device.
2. Description of the Related Art
Conventionally, image forming apparatuses such as printers and copying machines generally use cut sheets (copying paper) as a medium on which an image is formed, which cut sheets enable continuous supply. Hitherto, plain paper and woodfree paper specified by manufacturers of copying machines have been used as these sheets (paper). Plain paper and woodfree paper have low smoothness, and thus have weak adhesion. It is thus relatively easily to prevent so-called double feed wherein when sheets are to be supplied one by one from a sheet stacking section such as a sheet feed tray, a plurality of sheets are fed in an adhered state.
In recent years, however, as a result of the diversification of recording media, it has become necessary for various types of sheets including sheets having high surface smoothness to be conveyed. In particular, as coloring techniques are developed, versatile apparatuses that can also or alternatively convey media other than the conventional copying paper, such as enamel paper, the degree of whiteness of which is heightened and to which a glaze is applied (e.g., coated paper, and composite sheet where coating color is applied as one type of a coating material to both surfaces or one surface of a sheet in order to improve suitability for printing), film sheet or tracing paper, are in high demand.
Since these types of coated paper, film sheet and tracing paper have high surface smoothness, the adhesive force between sheets is strong, and thus it is difficult to prevent double feed thereof. For this reason, special measures are necessary for feeding such sheets. For example, the case of coated paper is explained below. As the quality of the coated paper becomes higher, the amount of coating increases, as does the optical property of white glossness. Further, unevenness on the surface of the paper is reduced so that the smoothness of the surface is increased. On the other hand, when the surface smoothness becomes high, the gap between contacting sheets becomes narrow, so that air does not pass through the gap.
As a result, negative pressure is generated and maintained, resulting in strong adhesion between sheets of coated paper. Particularly in cases where coated paper is stacked in a high-humidity environment, sheets of coated paper adsorb to each other so that the degree of adhesion becomes higher. Film sheet and tracing paper have high surface smoothness in themselves, and thus adhesive force between sheets thereof is naturally high and they exhibit the same characteristics in this regard as coated paper.
When these types of smooth sheets with strong adhesion between sheets are to be fed one sheet at a time from a sheet feed tray on which the sheets are stacked, it is very difficult to convey the sheets separately in a device that normally feeds normal paper. In a sheet supply device that uses an engaging roll which contacts with a surface of a sheet at a predetermined pressure and rotates so as to take-in the sheet and a system for pressurizing a supply roll and a separating roll at a constant pressure so as to separate the sheets when the sheets are sorted, a pressurizing force which is about 30 times as strong as that needed to separately convey normal paper is required for conveying smooth sheets separately.
As a method of eliminating the adhesion between the smooth sheets in advance, a technique for blowing air to side surfaces of the stacked sheets has been proposed (see, for example, Japanese Patent Application Laid-Open Nos. 3-211136 (1991) and 11-5643 (1999)).
FIG. 13A, for example, shows a case where air is constantly blown during feeding of the sheets, and FIG. 13B shows a case where an air blowing device is actuated intermittently in synchronous with the feeding of the sheets. In such cases, as shown in FIG. 13C, a sheet P is fed while rubbing against members above the sheet P, such as lift regulating members 564, a nudger roller 556 which has moved upward and a chute member 510, in a state in which an upward-pressing force for pushing up the sheet P is at a maximum. That is to say, the sheet P is fed while the surface thereof is rubbing hard against the lift regulating members 564, the nudger roller 556 and the chute member 510. For this reason, the surface of the sheet P is occasionally damaged.